Date: Thu, 7 Jan 1999 19:47:28 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Terry Lambert <tlambert@primenet.com> Cc: tlambert@primenet.com, pfgiffun@bachue.usc.unal.edu.co, freebsd-hackers@FreeBSD.ORG Subject: Re: questions/problems with vm_fault() in Stable Message-ID: <199901080347.TAA37034@apollo.backplane.com>
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:...........
:
:My personal preference for this would be to only expose the
:underlying FS layer through a notification layer -- disallow
:the notification layer from the underlying FS having promiscous
:exposure, and allow mounting on top of either a non-exposed OR
:a notification layer that may or may not be exposed.
The answer is that you need a general purpose cache coherency protocol
that you can count on to propogate throughout the VFS layering. *NOT*
an ad-hoc implemention in one VFS layer and another ad-hoc implementation
in another. What the VFS layer does on its backend -- if implementing
a network protocol such as AFS, NFS, CODA, or something like that, is
up to it. Those protocols may have their own cache coherency protocols
for their network interfaces, but is has very little to do with the
cache coherency protocol that needs to be implemented between VFS
layers.
:(2) MNFS manages distribute cache coherency across a network
: within the context of the existing framework.
MNFS is an externalized protocol, just as CODA and standard NFS
are. What they implement in their network layer is very different
from what they have to deal with in their VFS layer.
For example, lets say you export a UFS filesystem via MNFS. Ok, fine...
now lets say you import an MNFS filesystem and then re-export it to
another machine, and that machine imports it and then re-export
it to yet another machine. Will cache coherency be maintained across
the chain with MNFS? Without a general cache-coherency protocol for
inter-layer VFS it can't unless MNFS short-circuits the protocol.
Fine, so now mix protocols... you have a combination of MNFS, AFS, and
CODA mounts done in a chain. Lets say each one of these has cache
coherency. Will the coherency be maintained across the chain? Nope,
it won't. Not unless the VFS layer implements a general cache
coherency protocol that these filesystems use.
The real power of building a cache coherency protocol into the VFS
layering is that it allows you to do things you may not have thought of
yet.
-Matt
: Terry Lambert
: terry@lambert.org
Matthew Dillon Engineering, HiWay Technologies, Inc. & BEST Internet
Communications & God knows what else.
<dillon@backplane.com> (Please include original email in any response)
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